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TDD & Concurrency

Atomic Variables

Modern Processors have a single instruction

• Compare And Swap (CAS)
• Logical example of optimistic locking

IncrementValue() {
   int v;
   do {
      v = get current value
   } while(v != compareAndSwap(v, v+1))
}

• Keep attempting to increment value until it actually happens
• If processor does not support CAS operation, Java compiler simulates it.
  • If this is the case, high-contention systems will burn CPU time

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Conditions for Deadlock

• Mutual Exclusion
  • Multiple threads want to use same (limited) resource
  • Cannot be used at same time
  • E.g. A database connection
• Lock and Wait
  • Require multiple non-sharable resources
  • Grab one at a time
  • Hold until have them all
• No Preemption
  • One process cannot take another process’s resource
  • Or force another process to release resources
• Circular Wait
  • At least two resources required by at least two threads

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Avoiding Deadlock

• Break any of the 4 conditions
  • Makes deadlock impossible
  • Has performance and responsiveness ramifications
• Mutual Exclusion
  • Don’t lock mutual resources
  • Don’t share
  • Make more available
  • Only lock one at a time
• Lock and Wait
  • Don’t wait – back-off if everything not available
• No preemption
  • Demand threads to release
  • DO NOT use Thread.stop() – can leave resources locked
• Circular Wait
  • Order your locks

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